Method of and welding torch for arc welding

ABSTRACT

Plasma-MIG welding in which a thermally ionizable gas stream is flowed through a nozzle non-consumable electrode having a central orifice and a surrounding annular opening toward a workpiece and is thereby split into a central gas column enveloped by an annular gas shield. A consumable electrode is fed through the central gas column toward the workpiece, with the establishment of a MIG-arc therebetween. A plasma arc is then spontaneously established by means of the MIG-arc between the nozzle non-consumable electrode and the workpiece. The central plasma gas column is accelerated by constriction of the annular gas shield downstream of the nozzle non-consumable electrode.

This invention relates to a method of arc welding, wherein a MIG arc ismaintained between a consumable electrode and a workpiece in a gasplasma which is enveloped by a shielding gas, said gas plasma beinggenerated by a plasma arc between a non-consumable electrode and theworkpiece, a gas of the same composition being used as the plasma gas aswell as the shielding gas and being supplied as a common gas flow whichis split into a central column of plasma gas and a jacket of shieldinggas in the region of the non-consumable electrode.

A method of this kind is proposed in co-pending application Ser. No.862,008 filed Dec. 19, 1977. The latter method aims to improve thewelding process known from U.S. Pat. No. 4,016,397.

The present invention has for its object to increase the deposition rateof the consumable electrode, to improve the melting of the workpiece andthe material transfer from the consumable electrode, and to extend therange of application of such welding process.

This object is mainly achieved in accordance with the invention in thatthe flow rate of the gas plasma is increased by constricting theshielding gas, and hence the gas plasma, downstream of thenon-consumable electrode. Apparently, due to the constricting of theshielding gas jacket, part of the comparatively cold shielding gas isdriven into the plasma column where it is heated, so that it expands. Asa result, the flow rate of the gas plasma strongly increases. Theincreased flow rate of the gas plasma has two effects: the materialtransfer from the consumable electrode is strongly stimulated and thetransfer of heat to the workpiece is increased. The advantages achievedby means of this method will be elaborated hereinafter.

The transfer current intensity of the welding current through theconsumable electrode, beyond which the material transfer passes from adroplet transfer into a spray transfer, is substantially reduced, forexample, from 160 A to 100 A for a consumable electrode of aluminiumwire having a diameter of 1.2 mm. This readily results in a substantialimprovement of the material transfer at low current intensities throughthe consumable electrode, i.e. stronger ejection of finer droplets; thisis of major importance for welding "in position". For example, in thecase of welding vertically disposed plates, the welding current throughthe consumable electrode should not be too high, because otherwise themolten pool becomes too hot and too large and hence drips off.

The droplets are heated for a shorter period of time and the consumableelectrode is deposited faster, because a part of the energy whichotherwise would cause overheating, is now used for melting theconsumable electrode. For example, when aluminium wire is welded bymeans of the said, already known method, overheating is very high; at acurrent intensity of 160 A through the consumable electrode, a droptemperature of 1700° C. was measured; this means overheating by morethan 1000° C. Experiments with the method in accordance with theinvention have revealed an increase of the deposition rate of theconsumable electrode of 40% per ampere delivered by the current sourcefor the consumable electrode in comparison with the known method. Forwelding aluminium workpieces, a comparatively low temperature of thedrops is also particularly advantageous, because less hydrogen is thenabsorbed from the atmosphere surrounding the welding arc, so that fewerpores occur in the weld in comparison with the said known method.

It will be clear from the foregoing that the method in accordance withthe invention offers the following possibilities with respect to theknown method and depending on practical requirements: increaseddeposition rate of the consumable electrode for a given welding current,so that a welding gap can be more quickly filled at the same heat input;or deposition of a given quantity of welding metal at a lower currentintensity through the consumable electrode, and hence at a lowertemperature of the molten pool, notably in the case of welding "inposition"; or a combination of both possibilities, where obviouslyintermediate values are applicable as regards the deposition rate, thecurrent intensity and the temperature of the molten pool.

Due to the increased thrust of the gas plasma flowing at a high rate,the heat transfer from the hot gas plasma to the melting zone in theworkpiece is increased. Part of this increase of the heat transferoccurs because the liquid metal is blown away by the gas plasma, so thatthe solid bottom of the pool is exposed. For another part, the heattransfer from the quickly flowing gas plasma will be larger than from agas which flows slower. As a result, better penetration of the workpieceis obtained, which is favourable in order to achieve a smoothtransition.

It has also been found that, in spite of the constriction of theshielding gas jacket, the shielding of the plasma column by theshielding gas is maintained to a high degree. This is a very favourableside-effect which is important notably in the case of the welding ofaluminium, where shielding of the molten pool against ingress of air isobtained to an extent which has proven to be adequate in practice. Forwelding other materials, additional gas shielding may be used, ifnecessary.

In spite of the fact that a given quantity of weld metal of lowertemperature is deposited at a lower current intensity through theconsumable electrode, so that the dimensions of the molten pool aresmaller, the penetration of the workpiece increases. Moreover, afavourable method of droplet transfer is obtained. Notably welding "inposition" is substantially improved; due to the necessarily low currentintensity through the consumable electrode, the melting of the workpiecewith the welding processes known thus far is often marginal, so that"bonding" defects may occur.

It is to be noted that the constricting of a gas plasma per se isalready known from U.S. Pat. No. 2,847,555; however, the gas plasma isthen directly constricted by a reduced plasma orifice; any shielding gasbeing separately supplied and being of a composition other than that ofthe plasma gas, however, is not constricted; moreover, because theconsumable electrode is laterally introduced into the gas plasma, thedescribed advantages and effects, notably as regards the consumableelectrode, cannot be achieved or can only be partly achieved.

Workpieces welded by means of the method in accordance with theinvention are characterized by a comparatively narrow bead,comparatively deep penetration and absence of pores at the welded point.

The invention also relates to a welding torch for performing the saidmethod in accordance with the invention, comprising a housing with anozzle provided with a plasma orifice, a gas inlet, a contact tube inthe housing, and a gas conductor provided with an outlet aperture, thenozzle consisting of a central electrode ring which is connected to thehousing by radially extending ribs which form gas passage ductstherebetween. A torch construction of this kind is proposed in saidApplication Ser. No. 862,008; the welding torch in accordance with thepresent invention is characterized in that the cross-sectional area ofthe outlet aperture is smaller than the combined cross-sectional area ofthe gas passage ducts and the plasma orifice in the nozzle. This weldingtorch strongly constricts the shielding gas jacket and the plasmacolumn, so that the gas plasma is accelerated during its passage throughthe outlet aperture. The welding torch in accordance with the inventionis very simple, small and rugged, and is suitable for fully automaticand semiautomatic welding as well as for use as a hand torch.

The described welding torch is suitable for obtaining a constrictioneffect, and hence acceleration of the gas plasma, but only if a givenminimum quantity of gas is supplied to the welding torch. Because theratio of the cross-sectional area of the outlet opening to the combinedcross-sectional areas of the plasma orifice and the gas passage ducts issmaller than 1:1.3 and larger than 1:4 in a preferred embodiment of thewelding torch in accordance with the invention, a constriction effect,and hence an acceleration of the gas plasma, is obtained in allcircumstances.

A further preferred embodiment of the welding torch in accordance withthe invention is characterized in that the cross-sectional area of theoutlet aperture is equal to or smaller than the cross-sectional area ofthe plasma orifice. Thanks to this fact, the constriction effect of theoutlet aperture and the acceleration of the gas plasma can be influencedby variation of the quantity of gas supplied to the welding torch perunit of time.

When use is made of the welding torch known from U.S. Pat. No. 4,016,397gas quantities of from 15 to 40 l/min are supplied in practice. Whensuch gas quantities are supplied, optimum constriction and accelerationof the gas plasma are obtained in a further preferred embodiment of thewelding torch in accordance with the invention, in which the plasmaorifice and the outlet aperture have a circular cross-section, and inwhich the outlet aperture and the plasma orifice each has a diameter ofat least 6 mm and at the most 12 mm.

The invention will now be described in detail with reference to theaccompanying drawing, in which

FIG. 1 is a diagrammatic longitudinal sectional view of a welding torchin accordance with the invention;

FIG. 2 is a cross-sectional view of the welding torch, taken accordingto the line II--II in FIG. 1.

The welding torch 1 shown in FIGS. 1 and 2 comprises a housing 3 with anozzle 5 and a gas inlet 7. In the housing 3 there is arranged a contacttube 9 which serves for the current transfer to and the guiding of awelding wire 11 to be deposited. The contact tube 9 is insulated fromthe housing 3 by means of an insulating ring 13. The nozzle 5 consistsof a central electrode ring 15 which is provided with a plasma orifice16 and which serves as a non-consumable electrode for a plasma arc, asupport 17 which serves as a heat sink, and a number of radial ribs 19which are preferably regularly distributed around the circumference ofthe electrode ring 15 in a spider-like manner and which connect theelectrode ring 15 to the support 17.

The support 17 is mechanically, thermally and electrically connected tothe housing 3 by way of a screwed connection 21. The nozzle 5 issurrounded by a mainly cylindrical gas conductor 23 which is insulatedfrom the housing 3 by means of a ring 25 of a synthetic material andwhich surrounds the support 17 with clearance. A cooling water jacket 27is formed between the gas conductor 23 and the support 17, this jacketbeing sealed by means of sealing rings 29 which also serve for theelectrical insulation of the gas conductor 23 from housing 3. Thecooling water jacket 27 communicates with connections (not shown) forthe inlet and outlet of cooling water. The gas conductor 23 has agenerally conical end 31 which is provided with an outlet aperture 32and which extends, viewed in the axial direction, downstream of theelectrode ring 15. The welding wire 11 is supplied by means of transportrollers 33 which are driven by a variable speed motor 35. The contacttube 9 is provided with a connection terminal 37 for electricalconnection to one of the poles of a first power supply source 39, theother pole of which is connected to a workpiece W. The electrode ring 15is connected, by way of ribs 19, support 17, housing 3 and a connectionterminal 41 on the housing 3, to one of the poles of a second powersupply source 43, the other pole of which is also connected to theworkpiece W.

For the welding of the workpiece W, a gas flow G is supplied, via thegas inlet 7, which flows through the housing 3 in the direction of thenozzle 5. Subsequently, the welding wire 11 is fed and a MIG arc M isstruck between the welding wire 11 and the workpiece W, for example bybringing the welding wire into contact with the workpiece. A plasma arcbetween the electrode ring 15 and the workpiece W is then spontaneouslystruck by the MIG arc. In the nozzle 5, the gas flow G is split into twoparallel sub-flows by the electrode ring 15 and the ribs 19: namely acentral gas column flowing through the plasma orifice and which, afterthe striking of the plasma arc between the electrode ring 15 and theworkpiece W, is ionized to form the gas plasma P, and an annular jacketS of relatively cold, non-ionized sheathing or shielding gas which flowsthrough gas passage ducts 20 present between the ribs 19 and whichenvelops and surrounds the gas plasma P. The ribs 19, via which thecurrent is supplied to the electrode ring 15, also transfer heat fromthe electrode ring 15 to the support 17 which serves as a heat sink.

In accordance with the invention, the free sectional area of the outletaperture 32 is smaller than the combined sectional areas of the gaspassage ducts 20 and the plasma orifice 16. Preferably, the sectionalarea of the outlet aperture 32 is equal to or smaller than the sectionalarea of the plasma orifice.

Tests which were performed by means of a welding torch in which theoutlet aperture and the plasma orifice had a circular cross-section andwhere the above ratios were adhered to, offered excellent results fordiameters of the plasma orifice and the outlet aperture of between 6 mmand 12 mm; for the welding of aluminium and aluminium alloys, argon andmixtures of argon and helium were used, whilst for the welding of steeluse was made of mixtures of argon and CO₂ or oxygen, the suppliedquantities of gas varying from 15 to 40 l/min.

What is claimed is:
 1. A method of plasma-MIG welding, which comprisesproviding a single nozzle non-consumable electrode having a centralorifice and a surrounding annular opening; flowing a thermally ionizablegas stream from a common gas source through said single nozzlenon-consumable electrode toward a workpiece, said single nozzlesplitting said gas stream into a central gas column surrounded by aparallelly flowing annular gas sheath, said annular gas sheath and saidcentral gas column having the same composition as they flow past theoutlet of said single nozzle; feeding a consumable electrode throughsaid central gas column toward the workpiece; first establishing aMIG-arc between said consumable electrode and said workpiece; thenspontaneously establishing a plasma arc by means of said MIG-arc betweensaid single nozzle non-consumable electrode and said workpiece to ionizesaid central gas column and sustain a plasma flow enveloping saidMIG-arc, the surrounding annular gas sheath remaining non-ionized andrelatively colder than the resulting central plasma gas column; andconstricting the annular gas sheath downstream of the single nozzlenon-consumable electrode to drive the relatively colder gas sheath intothe central plasma gas column to expand the former by heating wherebythe central plasma gas column is also constricted and its flow rate isincreased.